Particulate solid dispersions are used in many fields including pharmaceuticals, agrochemicals, paints and especially inks. For many different reasons it is advantageous to provide colloidally stable dispersions. For aqueous dispersions achieving stability is more easy, not least because more dispersants are available which are well suited to aqueous liquid vehicles. For predominantly organic liquid media we have found it substantially more difficult to prepare colloidal stable dispersions. We have found that it is more difficult to prepare very fine dispersions of particulate solids in predominantly organic liquid vehicles. Thus, sub-micron sized particles are especially difficult to prepare in organic liquid vehicles.
Pigments (as particulate solids) may be used to prepare inks, paints, tints and especially ink jet printing inks.
Pigment inks tend to have better ozone-fastness and light-fastness than for example dye-based inks. However, because the pigment is in the form of a particulate dispersion (rather than a solution) there is a tendency for pigment particles to agglomerate or flocculate whilst the ink is being stored and/or whilst the ink is being used (e.g. printed). Such agglomeration or flocculation before the ink has been printed onto a substrate is highly undesirable, particularly in ink jet printing inks where the printer nozzles are very small and are susceptible to blockage by any oversized particulate matter. Thus, in the ink jet field a great deal of effort has been spent attempting to increase the colloidal stability of pigment dispersions.
We have found that it is particularly difficult to provide pigment dispersions and inks having good colloidal stability when the liquid vehicle is completely or predominantly comprised of organic liquids.
It is often difficult to adequately obtain a sufficiently fine (sub-micron) sized pigment dispersion.
It is also desirable to provide pigment inks in predominantly or completely organic liquid vehicles which offer high optical density (OD), especially when printed onto plain paper.
UV curable ink jet printing inks comprise sub micron sized pigment particles dispersed in a UV curable organic liquid vehicle. Examples of UV curable organic liquid vehicles include for example those comprising styrenic and/or (meth) acrylic monomers. It can sometimes be difficult to mill a pigment in such UV curable compositions as the time and energy required to prepare sub micron sized dispersions can cause premature reaction or gellation of the UV curable composition. Such pre reacted or gelled components can be very difficult to remove and can significantly impair the performance of ink jet printing inks made from such compositions.